And created a test image, creating black and white values directly on the red, green and blue channels, and erasing half the image as well to test the effects of extremes in all the different channels, and the results are...extremely confusing. Is there a more detailed guide on how to make detail maps for the new lit shader?

Unity Technologies

There is no guide currently, but I will try to compare it to how the Built-In standard detail maps works :
Basically, detail maps contain surface informations (albedo, normal, smoothness) that are added to the base surface information, in ordre to be used with a high tilling value.
In the Built-In standard shader, the detail maps are Albedo and Normal. In HDRP, we wanted to optimize the number of texture samplers in a material, and that's why there is only one map used for detail, where :

The red channel contains the albedo variation. It is applied to the underlying material in an "Overlay" blend mode, similar to photoshop. You can create this channel by simply desaturating the built-in detail albedo.

The green channel contains the normal Y value. Simply use the green channel of the built-in detail normal.

The blue channel contains the smoothness variation. One again, it is applied with an "Overlay" blending, and this can either be done by using the smoothness map of your detail material if you have one, or creating is from the albedo, or leaving it to a mid-gray value to keep the underlying smoothness unchanged.

The alpha channel contains the normal X. Like for the blue channel, you can simply use the red channel of the built-in detail normal.

Unity Technologies

Imagine that your base albedo is ... let say red.
The albedo variation in the detail map will keep the red information, but lighten or darken it. If the detail is mid gray, the final albedo is unchanged. It it is white, the final albedo will be white. And if it is black, the final albedo will be dark.
The behavior is similar for the smoothness.